This proposal highlights the importance of friends and romantic partners in the lives of adolescents. It explores the formation of friend pairs and romantic pairs, and how the composition of these pairs affects the subsequent events in the pair relationship. It explores the influence of the pair relationship on the health behavior and sexual behavior of members of the pair. It examines how the distribution of social characteristics in schools affects pair formation and the course of pair relationships. This proposal capitalizes on the central design features of the project that (1) identify friend pairs and romantic partners, (2) maximize the probability that both pair members will be in the study and provide behavioral data, and (3) match the data provided by pair members. The study uses data provided by pairs for a methodological analysis of the reliability and validity of responses about sensitive behaviors.